1. Field of the Invention
The current invention relates to communication networks capable of transmitting electrical power along with data, and more particularly, to systems and methods for the transmission of electrical power in Power-over-Ethernet (PoE) systems.
2. Description of the Related Art
A Power-over-Ethernet system is an Ethernet network capable of transmitting both data and electrical power over twisted wire pair cables, such as category 5 cables. Ethernet is currently defined by the IEEE 802.3 standard, and PoE is currently defined by the IEEE 802.3af standard, both of which are incorporated herein by reference. Using PoE allows for the convenient delivery of electrical power to Ethernet client devices, such as Internet telephones or cameras, which may otherwise require more cumbersome powering arrangements in order to operate. PoE allows for the delivery of electrical power using the same cables that transmit Ethernet data.
FIG. 1 shows a simplified block diagram of conventional power sourcing equipment (PSE) port 100, which is part of a PSE (not shown), which in turn is part of a PoE system that also comprises a powered device (PD) (not shown). The PD receives its electrical power from the PSE. A PSE can have multiple ports, wherein each port is adapted to connect to a PD. A PD can be, for example, a voice-over-IP (VoIP) telephone, a wireless hub, or a networked camera. PSE port 100 supports Ethernet communication services in accordance with the Ethernet standard, as well as the provision of electrical power in accordance with the PoE standard. Ethernet communication is supported by physical-layer module (PHY) 101, which connects to RJ45 interface 109 via signal-isolation transformers 113, 114, 115, and 116. RJ45 interface 109 in turn connects to the RJ45 interface (not shown) of the PD via a category 5 unshielded twisted pair cable path (not shown), which may include multiple cables and connectors. PHY 101 also connects to a media access controller (MAC) (not shown).
Transformers 113, 114, 115, and 116 support electrical isolation between the so-called isolated side of PSE port 100 and the so-called line side of PSE port 100. The isolated side is on the primary side of the transformers of PSE port 100, while the line side is on the secondary side of the transformers of PSE port 100. The two sides are isolated by an isolation barrier, represented in FIG. 1 as a dashed line, which helps protect sensitive devices on the isolated side from electrical surges on the line side. The isolation barrier is also supported by power transformer 107 and optical isolator 108. The isolated side of PSE port 100 comprises PHY 101, voltage supply 102, transistor 105, resistor 106, PSE switching regulator 103, and PSE controller 104. The line side of PSE port 100 comprises RJ45 interface 109, line-side PSE controller 110, diode 111, and capacitor 112. PSE port 100 further comprises devices supporting the isolation barrier, specifically, optical isolator 108 and transformers 113, 114, 115, 116, and 107, which are located on both sides of the isolation barrier. It should be noted that ports might share some components. For example, several ports might use the same voltage supply, PSE controller, or line-side PSE controller. Also, single physical components might support multiple ports, such as an octal PHY that can support eight ports. Thus, unless otherwise indicated or necessary, references herein and in the figures to particular elements refer to functional units and do not limit their physical implementation.
Voltage supply 102 provides electrical power for transmission to the PD, which expects to be able to draw a range of currents at a regulated voltage of 48V. Voltage supply 102 provides a DC voltage whose value is based on the supply voltages commonly available in the particular implementation of the PSE. An appropriate turns ratio is selected for transformer 107 so as to provide 48V to the PD. For example, voltage supply 102 can provide 48V with a turns ratio of 1:1 for transformer 107, or voltage supply 102 can provide 12V with a turns ratio of 1:4 for transformer 107. Other combinations of turns ratio and voltage are possible, as well. For example, voltage supply 102 can provide 12V and be paired with transformer 107 having a 1:5 turns ratio, allowing for voltage losses and/or regulation on the line side and/or in the PD. Electrical power from voltage supply 102 is (i) transformed by power transformer 107, (ii) conditioned by diode 111 and capacitor 112, and (iii) transmitted to the PD via line-side PSE controller 110 using two of the four pairs of wires in the cable path connecting the PD to PSE port 100. Although this specification describes Ethernet devices using four signal-isolation transformers and four wire pairs, the teachings of this specification apply equally well to devices using only two signal-isolation transformers and two wire pairs, or Ethernet devices using other isolation means.
The supply of electrical power from PSE port 100 to the PD is regulated by switching regulator 103 through use of transistor 105 and optional current-sense resistor 106. Switching regulator 103 uses path 105a to control transistor 105 by using pulse-width modulation (PWM). Typically, the frequency of the pulses generated by switching regulator 103 remains constant, but their width varies, thereby determining the duty cycle of the switching signal generated by switching regulator 103. The duty cycle is the ratio, for a cycle, of the interval that the pulse is high to the length of the cycle. Thus, a pulse cycle wherein the pulse is high for ⅗ths of the cycle has a 60% or 0.6 duty cycle. Generally, increasing the duty cycle increases the average output voltage provided to the PD.
If transistor 105 is on, i.e., during the high part of the duty cycle of the switching signal on path 105a, then current flows through the primary coil of transformer 107. If transistor 105 is on, then due to diode 111, substantially no current flows through the secondary coil of transformer 107. When transistor 105 is turned off, i.e., during the low part of the duty cycle of the switching signal on path 105a, current substantially stops flowing through the primary coil of transformer 107. Current then starts to flow through the secondary coil of transformer 107 as the electromagnetic energy built up in the primary coil of transformer 107 is transferred to its secondary coil. The current through the secondary coil starts at a level proportional to the current that was flowing through the primary coil, possibly exhibiting an initial spike, and either steadily declines all the way to zero, or drops to zero with the turn-on of transistor 105, which starts the cycle anew.
Switching regulator 103 can operate in either discontinuous or continuous mode, depending on the required current draw and the duty cycle. For a given regulated output voltage level, that is to say a given steady state duty cycle, at lower output current draws (light load), transformer 107 operates in discontinuous mode, in which the current through its secondary coil drops to zero during the phase of the switching cycle when the transistor 105 is turned off. For higher output current draws, transformer 107 operates in continuous mode, in which the current through the secondary coil of transformer 107 will not decline to zero before transistor 105 is turned on, at which point the current through the secondary coil drops to substantially zero.
Switching regulator 103 uses path 106a to sense the current through the primary coil of transformer 107 and transistor 105 by measuring the voltage across current-sense resistor 106. Switching regulator 103 can use the timing of fluctuations in the sensed current to determine the timing of the pulses for the generation of the PWM waveform. The actual values of the measured current are not used by switching regulator 103. Switching regulator 103 may also use other means to generate the PWM waveform. Switching regulator 103 may be controlled by PSE controller 104, which can receive information regarding power usage by the PD from line-side PSE controller 110, via optical isolator 108. Alternatively, switching regulator 103 can be set to provide a predefined switching signal, whereupon line-side PSE controller 110 can regulate the actual electrical power provided to the PD. PSE controller 104 communicates with a host controller (not shown) via path 104a. 
Line-side PSE controller 110 performs several PoE-related functions, including detection, optional classification, and fault monitoring. Detection comprises detecting whether a PoE-compliant PD has been connected to PSE port 100 by measuring currents and/or voltages. Classification comprises attempting to determine the expected power usage of the PD. After the detection of a valid PD, power is provided to the PD. Fault monitoring, which is used while power is being provided by PSE port 100 to the PD, comprises monitoring the level of current drawn by the PD to determine whether the PD has been disconnected, experienced a short-circuit, or started drawing too much power. Monitoring for PD disconnection is also known as maintain power signature (MPS) monitoring. If MPS is present (i.e., monitoring indicates that a power signature is being maintained), then the PD is considered connected.
Normally, if a PD is connected to PSE port 100, then PSE port 100 provides a variable electrical current, within a specified range, at a regulated voltage of 48V. If the level of the current drawn by the PD falls below IMIN, a specified minimum threshold, for at least a specified interval, then it is determined that the PD has been disconnected, and PSE port 100 cuts off the provision of electrical power to the PD. The present standard specifies that, at electrical current levels of less than 5 mA, MPS is absent, while, at electrical current levels of more than 10 mA, MPS is present. For electrical current levels between 5 mA and 10 mA, the determination regarding the presence of MPS is dependent on the particular implementation. The present standard further specifies that the PD may not be disconnected if MPS is absent for less than 300 mS, and must be disconnected if MPS is absent for more than 400 mS. Action for MPS absences of between 300 and 400 mS is implementation-dependent. Following the cut-off of power, i.e., disconnection, PSE port 100 returns to detection mode, to determine whether a valid PD has been connected.
PSE port 100 also monitors the electrical-current usage of the PD for other fault conditions to prevent power overloads and excessive current draws. Current draw may never exceed a maximum current ILIM, set in the present standard at between 400 and 450 mA. A current draw greater than ILIM is treated as a short circuit and causes a cut-off of electrical power. To prevent power overloads, the present standard requires disconnecting the PD if current draw exceeds a threshold value ICUT, for more than a threshold time period TCUT. The present standard specifies an ICUT of 350-400 mA, and a TCUT of 50-75 mS. The present standard further specifies that, if current draw exceeds ICUT for TCUT, then power will be disconnected to the PD for a “penalty” period of 3-5 seconds.
Line-side PSE controller 110 can communicate with PSE controller 104 through optical isolator 108. Optical isolator 108 comprises a light-emitting diode (LED) and a phototransistor or photodiode for electrically-isolated transmission of information from the line side to the isolated side.